FIG. 13 is a block circuit diagram of a conventional electric power converter including power semiconductor modules. In FIG. 13, a DC power supply 1, power semiconductor modules 2U, 2V and 2W, a motor 3 as a load, and a driver circuit 4 for driving power semiconductor modules 2U, 2V and 2W for the respective phases are shown. Each power semiconductor module 2U, 2V or 2W includes two insulated gate bipolar transistors (hereinafter referred to as “IGBTs”) connected in series and two free wheel diodes connected in opposite parallel to the respective IGBTs. Power semiconductor modules 2U, 2V and 2W for the respective phases switch on and off the respective IGBTs on the upper arm and the respective IGBTs on the lower arm alternately to convert the AC electric power from DC power supply 1 to AC electric power and to supply the converted AC electric power to motor 3.
Pulse width modulation (hereinafter referred to as “PWM”) has been well known as one of the methods for controlling the IGBTs′ switching. A control circuit 5 including a comparator 5c is disposed outside power semiconductor modules 2U, 2V and 2W for conducting PWM control. Comparator 5c compares a reference output voltage 5a and a carrier 5b and determines a switching pattern. The determined switching pattern is sent to driver circuit 4. Driver circuit 4 converts the switching pattern to gate signals and sends the gate signals to respective modules 2U, 2V and 2W.
Power semiconductor devices such as the IGBTs and the free wheel diodes are mounted on packages for the respective phases such that power semiconductor modules 2U, 2V and 2W are configured. The power semiconductor module configuration as described above facilitates simplifying the electric power converter structure, assembling the electric power converter easily, wiring the electric power converter easily, and cooling the devices in the electric power converter.
FIG. 14 is a schematic cross sectional view of power semiconductor modules 2U, 2V or 2W. Since each of the power semiconductor modules 2U, 2V and 2W have the same structure, the power semiconductor module is designated in FIG. 14 by the common reference numeral 2. As shown in FIG. 14, circuit patterns 9 are formed on a ceramic insulator substrate 16. Semiconductor chips 8 of the IGBTs and the free wheel diodes are mounted on respective circuit patterns 9. Circuit patterns 9 and semiconductor chips 8 are connected to each other via electrically conductive wires 7 made of aluminum or some other such electrically conductive material. Circuit patterns 9 are connected to an input terminal 2a or 2b connected to DC power supply 1. In FIG. 14, impedance elements 6y, which will be described later in detail, are shown. Impedance elements 6y are fixed to respective input terminals 2a and 2b. An insulator substrate 16 is mounted on the upper surface of a radiator plate 10 made of a copper alloy. A cooling fin 11 is fixed to the back surface of radiator plate 10 for dissipating the losses (heat) caused by semiconductor chips 8 in the electrically conductive states thereof to air. The cooling fin 11 is connected usually to the ground through an earth line 12 to prevent the electrification caused by a contact to cooling fin 11 from occurring.
In the above described conventional electric power converter including power semiconductor modules 2, excessively large switching noises are caused by the switching of the IGBTs and such power semiconductor devices. The excessively large switching noises further cause malfunctions of the other apparatus and equipments disposed around the electric power converter, noises in the other apparatus and equipments and such hazards. The switching noises may be generally classified into two different kinds: normal mode noises and common mode noises.
The normal mode switching noises are noises caused by the normal-mode high-frequency current that flows through the closed loop consisting of power semiconductor module 2 and DC power supply 1. The closed loop 13a, through which the normal mode noise current flows, is shown in FIG. 15(a). In the normal mode, LC resonance is caused by the switching of the power semiconductor devices based on the floating inductance of the wiring constituting closed loop 13a and the junction capacitance of the power semiconductor devices, further causing a high-frequency noise current that flows through closed loop 13a. 
The common mode switching noises are noises caused by the noise current that flows through earth line 12 via the floating capacitance (or the earth capacitance) of power semiconductor module 2 and the floating capacitance (or the earth capacitance) inside the electric power converter. The closed loop 13b, through which the common mode noise current flows, is shown in FIG. 15(b). In FIG. 15(b), the floating capacitance 14 caused by the wiring in the electric power converter, the floating capacitance 15a and the floating capacitance 15b of the power semiconductor module 2 are shown.
In the common mode, floating capacitance 14, floating capacitance 15a and floating capacitance 15b are charged and discharged at a high frequency by a high voltage change (dV/dt) caused by the switching of the power semiconductor devices, causing a high-frequency charging and discharging current that flows through closed loop 13b via earth line 12. In some cases, the common mode noise current flows out to the side of DC power supply 1 or the common mode noises are radiated in the form of radio waves.
An impedance element, such as inductance, has generally been added to the noise current loop to suppress the normal mode noise current and the common mode noise current. In FIG. 14, for example, impedance elements 6y are connected in series to respective semiconductor chips 8 for suppressing the normal mode noise current.
As described above, the normal mode noise current and the common mode noise current will be suppressed to some extents, if an impedance element is disposed in the noise current path. If the impedance element for suppressing the noise current is arranged as closely as possible to the power semiconductor device, the entire circuit size may be reduced effectively and the packaging may be conducted effectively.
Conventional methods for suppressing various kinds of noise currents and the electromagnetic waves caused by switching are disclosed in the patent documents described below.
Unexamined Published Japanese Patent Application 2000-58740 discloses a suppressing means for suppressing common mode noises. The suppressing means includes a ring-shaped filter element made of a composite magnetic material and surrounding the positive and negative poles of an inner lead (connection line) for connecting a DC power supply to a semiconductor device (IC chip). The filter element is sealed in a package.
Unexamined Japanese Patent Application Hei. 9 (1997)-121016 discloses a noise reducing element made of an amorphous magnetic alloy. The noise reducing element is set around the lead portion of a diode and such a semiconductor device or molded into a unit with the lead portion of a diode and such a semiconductor device.
Unexamined Published Japanese Patent Application Hei. 11 (1999)-238844 discloses a ring-shaped soft magnetic element made of an amorphous alloy or a micro-crystalline magnetic alloy and disposed around a semiconductor device.
Unexamined Published Japanese Patent Application 2001-160605 discloses an electromagnetic shield structure that surrounds the edges of a semiconductor package substrate, mounted on a printed circuit board by the melting of solder balls and by the bonding with the molten solder balls. The electromagnetic shield structure surrounds the edges of a semiconductor package substrate with a ferrite cap or with an electromagnetic wave absorber such as moldings and a tape made of an electromagnetic wave absorptive resin so that the electromagnetic waves radiated from the junction portions between the solder balls and printed circuit board may be absorbed.
Since the conventional technique disclosed in Unexamined Published Japanese Patent Application 2000-58740 (hereinafter referred to as the “conventional technique 1”) seals the filter element made of a composite magnetic material in a package, it is impossible to replace the filter element (to change the inductance value). Usually, the switching noise frequencies vary depending on the surroundings' circuit conditions (such as wiring floating inductance and floating capacitance). Due to this, it is desirable to dispose a filter element, added to a semiconductor device for noise reduction, outside a package so that the filter element may be replaced, if necessary. However, it is impossible for the conventional technique 1 to replace the filter element, even if it is necessary. Since the conventional technique 1 houses the filter element in a package, the package is inevitably large. These problems are also posed on the conventional technique described in FIG. 14.
Since the conventional technique disclosed in Unexamined Japanese Patent Application Hei. 9 (1997)-121016 (hereinafter referred to as the “conventional technique 2”) assumes the application thereof to a low-power semiconductor device having lead wires, it is impossible to apply the conventional technique 2 to a middle- or high-power semiconductor module without modification nor improvement. Since large wiring parts such as wide area copper bars are connected to the power module terminal usually to make a high current flow, it is impossible to set the noise reducing element as described in the reference around the power module terminal.
Since the conventional technique disclosed in Unexamined Published Japanese Patent Application Hei. 11 (1999)-238844 sets a ring-shaped soft magnetic element around a semiconductor device in almost the same manner as the conventional technique 1, the desired noise reduction effects may not be obtained, if the peripheral circuit conditions are unfavorable.
When an electromagnetic shield is formed using a cap by the conventional technique disclosed in Unexamined Published Japanese Patent Application 2001-160605 (hereinafter referred to as the “conventional technique 4”), it is necessary to provide the inner surface height of the cap with a certain leeway, since variations are caused between the molten states of the solder balls. Therefore, it is troublesome to manage the cap size and to design the cap. The cap is shaped with a cover having a window and manufactured through many manufacturing steps, causing high manufacturing costs. It is hard to replace the moldings or the tape used for the electromagnetic wave absorber according the conventional technique 4, even when it is necessary to replace the electromagnetic wave absorber corresponding to the frequencies of the noises to be absorbed.
In view of the foregoing, it would be desirable to provide a power semiconductor module that facilitates replacing the magnetic element such as a ferrite core (hereinafter referred to as the “magnetic core”) thereof and reducing the size thereof. It would be also desirable to provide a power semiconductor module that facilitates setting a magnetic core around the module package securely. It would be further desirable to provide a power semiconductor module provided with a magnetic core having a relatively simple structure and easy to manufacture.